In U.S. Pat. No. 4,201,052, incorporated herein by reference, there is disclosed a pilot pressure operated high pressure load sensing valve system incorporated in a valve body designed to be mounted directly on an actuator to be controlled such as a hydraulic cylinder or hydraulic motor. The valve system accurately controls the position and speed of operation of the actuator.
In brief, the valve system disclosed in the aforementioned patent comprises an independent pilot operated meter-in element; a pair of load drop check valves; a pair of independently operated normally closed meter-out elements; a pair of load pressure responsive valves; and a pair of anti-cavitation valves. The meter-in element functions to direct fluid flow to one or the other of the actuator ports. The normally closed meter-out elements are associated with each of the actuator ports for controlling fluid flow from the port opposite to the actuator port to which the meter-in element is directing fluid. The meter-out elements function as variable orifices metering fluid between the appropriate actuator port and a low pressure zone such as a reservoir tank. Each of the meter-out elements has associated therewith the load pressure responsive valves which act on the meter-out elements in response to load pressure to enable the meter-out elements to also provide pressure relief protection. The anti-cavitation valves are associated with each of the actuator ports and are adapted to open the appropriate port to tank.
The valve system is directly mounted to the actuator port manifold and is supplied by one full flow high pressure line, a pair of pilot pressure lines, and a load sensing line. The operation of the valve system is controlled through the pilot lines from a manually operated hydraulic remote control valve. In the absence of a command signal from the hydraulic remote control, the meter-in element assumes a centered or neutral position with the check valves, the meter-out elements, the pressure responsive valves, and the anti-cavitation valves, all in closed position. In the neutral position, the valve system prevents uncontrolled lowering of loads and in the case of overrunning loads, prevents fluid flow from the high pressure fluid source to the actuator even in the event of a ruptured line. Since the valve system is a load sensing system, the pump output is made to match that which is required by the load. In contrast, in a non-load sensing system, the pump output may exceed that required by the load with the excess power being dissipated as heat.
Under certain conditions, it may not be possible or desirable to mount the valve system directly to an actuator. Such conditions may exist due to space limitations on the actuator or where it is desirable to limit the number of supply and pilot lines, such as to the topmost section of a telescoping boom or when a brake, such as in a winch-type application, is required between the actuator and valve system. Under these conditions, the valve system is mounted on the equipment remote from the actuator with a pair of lines running to the actuator port manifold. In one of these situations, it may be desirable to interpose a conventional counterbalance valve between one of the actuator port lines and the valve system. The counterbalance valve provides for controlled lowering and holding of the load at the actuator port manifold.
In another situation when a stable load is involved, it may be desirable to interpose a pilot operated check valve between the actuator port and the valve system. The pilot operated check valve provides for positive holding of the load, that is, holding the load stable with zero drift.
Also, in many applications, the need arises for a linear hydraulic cylinder to have a float position or a rotary hydraulic motor to have a free swing or coast position. In either of these applications, the implement at the end of the cylinder or a swing drive for a boom are allowed to coast to a stop due to frictional forces in the system.
The valve system disclosed in the aforementioned patent does not lend itself to use in the circuit applications mentioned above; namely, the use of counterbalance valves, pilot operated check valves, brakes and free float or swing of the actuator. This is mainly due to the normally closed condition of the meter-out valve elements.
Accordingly, it is an object of the present invention to provide a valve system of the aforementioned type which is operable with the use of counterbalance valves, pilot operated check valves, brakes and free floating or swinging actuators.
In accordance with the invention, the above described control valve system is provided with a pair of normally open exhaust valves positioned between a tank passage and actuator ports so that with the meterin valve in the neutral position, both actuator ports are open to the tank passage through the normally open meter-out valves and the actuator will be free to move as, for example, in the case of a free coasting boom. However, when a pilot signal is applied to the meterin valve to move the actuator in one direction, pilot pressure is also applied to close the appropriate exhaust valve preventing flow of fluid from the pump to the tank passage with the other exhaust valve remaining open to the tank passage. Where a counterbalance valve is utilized in association with one opening of an actuator for controlling lowering and holding of a load, a single normally open exhaust valve is provided between that actuator opening and the tank passage. Where an external brake is provided for holding a load, a single normally open exhaust valve is also provided between the actuator opening and the tank passage.
In accordance with another aspect of the invention, the control valve system is of the type described above with reference to U.S. Pat. 4,201,052 utilizing a pair of normally closed meter-out valves. Where a free coast function is desired, a passage is provided from the output of the meter-in valve such that when a pilot signal is provided to the meter-in valve to apply pressure to one actuator opening to move the actuator in one direction, the exhaust pressure from the other actuator opening through the passage functions to open the meter-out valve associated with the other opening of the actuator to open that meter-out valve and provide a free coast mode.